492 Thermal Transpiration and Radiometer Motion. 



motion is reversed, which is not the case with mica and pith 

 (of course we are speaking of forms in which two or more 

 vanes are arranged symmetrically with regard to the pivot) ; 

 this reversal simply shows that the metal is a better absorber 

 of dark heat than the lampblack. 



Bnt in working with vanes made of gold-leaf Crookes 

 noticed that while the blackened side of one vane appeared to 

 be repelled by a candle, that of another appeared to be 

 attracted, and on examination it turned out that while the 

 former vane was flat the latter was crumpled and bent in 

 such a manner as to present a concave surface to the light. 

 Following up this clue by constructing radiometers with bent 

 and curved vanes Crookes was able to prove that in radiometer 

 motion shape of the vane exercises even more influence than 

 the absorbing power of the surface, so that a convex bright 

 surface appears to be strongly repelled by a source of light, 

 while a black surface if made concave to the light is actually 

 attracted by it. 



The theory of curved vanes is simple : consider a convex 

 vane irradiated by a source on the normal through its middle 

 point ; then, as the amount of heat that a surface absorbs 

 depends on its obliquity to the incident radiation, the farther 

 a part of the convex surface is from the middle the less is it 

 directly heated, and thus there is a continuous fall of tempera- 

 ture from the centre of the surface to the edge ; conduction, 

 if allowed time, tends to reduce the amount of the fall but 

 does not obliterate it, and conduction also establishes a fall of 

 temperature along the back from centre to edge ; now the 

 traction of the gas on the solid is from hot to cold, so that 

 both on the front and the back ot the vane there is a traction 

 from centre to edge whose resultant effect is to drag the vane 

 away from the light when the vane is convex to it, so that the 

 light appears to repel a convex surface ; when the surface is 

 concave the same reasoning applies, the gas exerts a traction 

 from centre to edge, and therefore the light appears to 

 attract it. 



There is hardly any need to reproduce any more of Orookes's 

 facts or Pringsheim's skilful experimental analysis of the 

 parts played in radiometer motion by bulb, vane, and gas ; 

 enough has been given to show that the kinetic theory can 

 account qualitatively and quantitatively for all the essential 

 facts of radiometer motion and furnishes general principles 

 for the design of apparatus of the radiometer type. An 

 illustration of the application of these principles will be given 

 in a separate paper on "Two New Pressure- Gauges for the 

 Highest Vacua." 



Melbourne, August 1896. 



